The patented bleeder screw with the built-in check valve. Once installed, it becomes a permanent part of the brake system. When it is time to bleed your brakes, you loosen the Speed Bleeder 1/4 - 1/2 turn and pump your brake pedal. When the pedal is depressed, the pressure generated opens the check valve letting air and brake fluid out of the end of the Speed Bleeder. When you release the pedal to the up position, the check valve closes and prevents any air from re-entering. When air-bubble free brake fluid is evident - close the Speed Bleeder. Makes bleeding your brakes much easier! Sold in Pairs.